In recent years, there have been proposed various kinds of optical elements in which an antireflection treatment for reducing reflection of light is performed to a surface. Antireflection treatments include, for example, a treatment in which an antireflection film is formed of a film (low refractive index film) having a relatively low refractive index, a multilayer film including a low refractive index film, and a film (high refractive index film) having a relatively high refractive index which are alternately stacked, or the like on a surface of an optical element (see, for example, Patent Document 1 and the like).
However, to form a low refractive index film or an antireflection multilayer film formed of a multilayer film of this type, a complicated step, such as vapor deposition, sputtering or the like, has to be performed. Therefore, there arises a problem that productivity is low and cost is high. Moreover, there is another problem that a low refractive index film or an antireflection film formed of a multilayer film exhibits large dependence on wavelength and incident angle.
In view of the above-described problems, as an antireflection treatment having relatively less dependence on incident angle and wavelength, for example, a treatment in which a fine structure is formed on an optical element surface so that concaves/convexes are regularly formed with a pitch equal to or smaller than a wavelength of incident light has been proposed (see, for example, Non-Patent Document 1, and the like). With this treatment, abrupt change in refractive index at an interface can be reduced, and a moderate change of refractive index is formed at the interface. Accordingly, reflection at the optical element surface is reduced, so that a high rate of incidence of light coming into the optical element can be obtained.
Patent Document 2 discloses a technique of forming fine concave/convex portions on a rough surface.